SF mayor’s race depends on ranked-choice voting —...

Amy Farah Weiss, Jane Kim, Mark Leno, moderator Melissa Caen, London Breed, Ellen Lee Zhou, Angela Alioto and Richie Greenberg during San Francisco mayoral debate at Commonwealth Club in San Francisco, on May ... more

Amy Farah Weiss, Jane Kim, Mark Leno, moderator Melissa Caen, London Breed, Ellen Lee Zhou, Angela Alioto and Richie Greenberg during San Francisco mayoral debate at Commonwealth Club in San Francisco, on May 14, 2018. less

Photo: Scott Strazzante / The Chronicle

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Voters during lunch at San Francisco City Hall voting center on May 24.

Photo: Liz Hafalia / The Chronicle

The special election to decide who will be San Francisco’s next mayor is shaping up to be the first contest for the city’s top job where the outcome hinges on the ranked-choice system, which voters adopted in 2004.

Under ranked choice, if no candidate receives the majority needed to claim outright victory, voters’ second and third selections become critical.

Here’s how it works:

•Voters select their top three picks for mayor, in order of preference.

•If no one receives a majority of votes in the first round — at least 50 percent plus one — the candidate with the fewest first-choice votes is eliminated, and his or her votes are redistributed to supporters’ second choices.

•That process continues again and again, until a candidate reaches the 50 percent plus one threshold and is declared the winner. There are eight candidates for mayor on the ballot Tuesday.

Voter Guide

To help explain the ranked-choice system, San Francisco’s Department of Elections created a video breaking down the vote-counting process using colored sticky-notes to represent candidates.

Depending on just how close the election is, it will likely take days to determine a winner. John Arntz, director of the city’s Department of Elections, said there are usually between 30,000 and 35,000 vote-by-mail ballots dropped off at polling locations on election day and perhaps another 10,000 ballots that have to be sorted when people vote at the wrong location.

On top of that, the Elections Department has to process ballots sent by mail that arrive after Election Day. If they’re postmarked on or before June 5, they can still be counted.

SF Mayor's Race

But a clearer picture of the likely winner should start to emerge by Saturday, June 9, or the following day, Arntz said, when most of the vote-by-mail ballots dropped off at polling places have been counted.

Dominic Fracassa covers San Francisco City Hall for The Chronicle. He previously worked as a reporter and editor for the Daily Journal, a legal affairs newspaper. He started in news in his home state of Michigan, where he worked as a news director of 103.9 WLEN.